Why it matters to a buyer
Duplicate content is rarely a deliberate attempt to cheat; it is usually an accident of how a platform generates URLs, print versions, filtered category pages, http and https both live, a www and non-www version both crawlable, and each variant silently competes with the page a business actually wants to rank. A buyer should know that Google's own guidance says duplication is not penalised as spam in the ordinary case; the real cost is wasted crawling and Google picking the wrong version as canonical, not a punishment.
How it is measured or applied
Duplication is found with a site crawl that groups pages by near-identical content and by checking whether known variants, parameters, protocols, trailing slashes, all resolve to one consistent version. Fixes are structural: 301 redirects where one URL should simply not exist, canonical tags where both versions have a legitimate reason to exist, and consistent internal linking to the preferred URL throughout the site. Search Console's page indexing report shows which URLs Google treats as duplicates and which canonical it chose, which is the most direct way to confirm a fix actually worked.
Sources
- What is URL canonicalization, Google Search Central
